In recent years, an electronic device (compound semiconductor device) in which a GaN layer and an AlGaN layer are sequentially formed on a substrate formed of gallium nitride (GaN) or Si and the GaN layer is used as an electron transit layer is actively developed.
The band gap of the GaN is 3.4 eV, which is greater than 1.1 eV of Si and 1.4 eV of GaAs. Therefore, the compound semiconductor device is expected to perform an operation with high breakdown voltage.
One of such compound semiconductor devices is a GaN-based high electron mobility transistor (HEMT). Hereinafter, the GaN-based high electron mobility transistor is referred to as GaN-HEMT. The HEMT is a field effect transistor in which a high-mobility two-dimensional electron gas (2DEG) induced by semiconductor heterojunction is used as a channel.
When the GaN-HEMT is used as a switch of a power supply inverter, it is possible to reduce ON resistance as well as improve voltage resistance. Further, it is also possible to reduce standby power consumption and improve operating frequency as compared with a Si-based transistor.
Therefore, it is possible to reduce switching loss, so that the power consumption of the inverter may be reduced. The GaN-HEMT may be smaller than the Si-based transistor having the same performance as that of the GaN-HEMT.
When the GaN-HEMT is operated at high frequency and high voltage, a current collapse phenomenon occurs in which the drain current decreases. One of the causes of the current collapse phenomenon is assumed that free electrons are trapped at the electron trap level of a region near a side of the gate electrode, which faces the drain electrode. When the electrons are trapped at the trap level of the surface, the density of the 2DEG decreases and the output of the GaN-HEMT may decrease. As a countermeasure against the current collapse phenomenon, there is a GaN-HEMT with a source field plate, in which a field plate is provided to the source electrode.
However, there is a problem that the threshold value varies when the GaN-HEMT is OFF, a high voltage is applied to an insulating film below the source field plate, the insulating film is deteriorated, and the life of the GaN-HEMT is shortened.
The following is reference document:
[Document 1] Japanese Laid-open Patent Publication No. 2006-324839.